The forcing together of soil particles under pressure, usually by foot or vehicle traffic. Compaction decreases soil porosity and increases bulk density. The degree of compaction is determined by the amount of pressure applied and soil characteristics, including clay, water, and organic matter contents. Although compaction sometimes can be beneficial by improving seed and root contact with the soil and by increasing the soil's ability to hold water, most compaction is detrimental to plants and soil animals. It can destroy soil structure, decreasing water intake, percolation, gas exchange, and biological activity, while increasing water runoff, erosion, and resistance to root penetration. Soil compaction often is a serious problem in agricultural fields, forests, range lands, lawns, and golf courses. Conversely, soil compaction is necessary for most construction purposes.